In recent years, the “closed draw” papermaking machine has been developed for achieving higher speed operation of a papermaking machine. In contrast with the conventional open draw machine, in which a wet paper web is transferred without being supported, in the closed draw machine, the wet paper web is supported throughout the papermaking process. The closed draw structure solves various problems encountered in the operation of the open draw machines, such as running out of paper. Thus, higher speed production can be achieved.
In a typical closed draw papermaking machine, as depicted schematically in FIG. 9, a wet paper web WW, shown by a broken line, which is transferred from right to left in the figure, is supported by press felts PF1 and PF2, a wet paper web transfer belt TB, and a dryer fabric DF. The press felts PF1 and PF2, the wet paper web transfer belt TB, and the dryer fabric DF, are endless belts supported by guide rollers GR.
The wet paper web WW passes through a press part comprising a press roll PR, a concave shoe PS, which conforms to the shape of the press roll, and a shoe press belt SB. The wet paper web then moves past a suction roll SR. The press part and the suction roll structures are generally known.
In the operation of the closed draw machine, a continuous wet paper web WW passes successively through a wire part and a first press part. (The wire part and the first press part are not illustrated.) The wet web is carried from the first press part on press felt PF1, and is then transferred to press felt PF2, as shown in FIG. 9. The press felt PF2 transfers the wet paper web to the press part PP. The wet paper web WW is pinched between by the press felt PF2 and the wet paper web transfer belt TB by the pressure applied by the press roll PR, and by the shoe PS through the shoe press belt SB.
The press felt PF2 has high water permeability and the wet paper web transfer belt TB has low water permeability. Consequently, water in the wet paper web WW moves to the press felt PF2 at the press part PP.
Immediately after the press felt PF2, the wet paper web WW, and the wet paper web transfer belt TB move out of the press part, the pressure on them is suddenly released, and they expand in volume. This expansion, together with the capillary action of the pulp fibers forming the wet paper web WW, causes a rewetting phenomenon wherein part of water in the press felt PF2 moves back into to the wet paper web WW. However, since the wet paper web transfer belt TB has very low permeability, it does not hold water. Therefore, rewetting from the wet paper web transfer belt TB does not occur, and the transfer belt TB contributes to improvement in the efficiency of water removal from the wet paper web.
After the wet paper web WW moves out of the press part PP, it is transferred by the transfer belt TB to the suction roll SR, where the wet paper web is transferred to dryer fabric DF which carries the web through a drying process.
There are several requirements for the proper operation of the wet paper web transfer belt TB. For transfer, the wet paper web WW must be attached to the transfer belt TB, during transport, after the belt moves out of the press part PP. However, the wet paper web WW must be removable from the transfer belt TB smoothly when the web is transferred to the next stage of the papermaking process.
Various transfer belt structures have been proposed for meeting these requirements. Among them, is belt structure of FIG. 10, which is described at pages 7 and 10–13, and shown in FIG. 4, of Japanese Patent No. 3264461. In FIG. 10, the wet paper web transfer belt TB10 comprises a woven fabric 31, a high molecular weight elastic section 51 formed on one side of the woven fabric, and a batt layer 41 formed on the other side of the woven fabric. The wet paper web side layer TB11 of belt TB10 is formed by the high molecular weight elastic section 51 and the machine side layer TB12 is formed by the batt layer 41.
The exposed surface of the wet paper web side layer TB11 is roughened by grinding. The ten-point average surface roughness Rz (according to JIS-B0601) is in the range of 0 to 20 microns while the belt is in the press part, and in the range of 2 to 80 microns after the belt moves out of the press part.
The ten-point average roughness Rz, in the range of 0 to 20 microns in the press part is maintained for a short time after the belt moves out of the press part. In other words, the surface of the wet paper web side layer TB11 is smooth at this point. Therefore, a thin film of water may be formed between the wet paper web and the smooth surface of the wet paper web side layer TB11. The thin film of water causes the wet paper web to adhere to the surface of the wet paper web side layer TB11.
As the transfer belt TB10 travels away from the press part, the ten-point average surface roughness of its wet paper web side layer TB11, increases to a level within the range from 2 to 80 microns. The increase in the surface roughness of layer TB11, breaks the thin water film, reducing the adhesion between the transfer belt and the wet paper web. Therefore, the wet paper web can be more easily transferred from the belt TB10 to the next stage of the papermaking process.
The transfer belt shown in FIG. 10 meets the requirements described above for the proper operation of a wet paper web transfer belt by continually changing its surface roughness as it passes through the press part of the papermaking machine. However, in use the wet paper web side layer TB11 becomes worn, and the desirable effects resulting from the changing surface roughness of the belt diminish. Consequently, the belt becomes increasingly difficult to use over time.
To address this deficiency in the belt of FIG. 10, Japanese Patent No. 3264461 discloses an alternative transfer belt structure, as shown in FIG. 11, in which particles 60 of a filler protrude from the surface on the wet paper web side layer TB11. For the purpose of illustration, the size of the filler particles is exaggerated in FIG. 11, since the actual particle size is in the order of a micron. The protruding filler particles 60 contribute to breaking of the thin water film. Moreover, the use of a hydrophilic filler makes it possible for the thin water film which is formed after the belt moves out of the nip of the press part to concentrate at the locations of the protruding filler bodies 60 and thus be destroyed.
Kaolin clay (hydrous silicic acid aluminum, having the general chemical formula Al2O3.2SiO2.2H2O) is used for the filler.
Because the surface of the wet paper web side layer TB11of the transfer belt is relatively smooth, there is a high likelihood that some filler bodies will separate from the surface of the belt, either during manufacture of the belt or during its use in the papermaking process. In the manufacturing process, the filler which is mixed with liquefied high molecular weight elastic material, and the mixture is applied to a woven cloth 31 and then cured. After curing, the surface of the wet paper web side layer TB11 is ground, and in the grinding process some of the filler is scooped out. Filler can also separate from the belt in the papermaking process due to the high operating speeds and the strain in the belt resulting from the application of pressure in the press part of the machine. Because of the loss of filler, it has been difficult to obtain uniform physical properties in a transfer belt, and adequate durability. Thus it was difficult to produce a wet paper web transfer belt suitable for use over a long time.
An object of this invention, therefore, is to provide a wet paper web transfer belt which can be used for a long time, while fully meeting the requirements for attachment of the wet paper web to the belt during transport, and smooth removal of the wet paper web from the belt when the web is transferred to a next stage in the papermaking process.